Gene therapy is a nascent medical technology that involves the insertion of genes into human cells and tissues to treat diseases, particularly hereditary diseases in which mutant genes are replaced with functional healthy ones. The first approved gene therapy procedure was performed as early as 1990.

In the past thirty years the procedures and techniques for carrying out basic gene transfers have improved considerably due to the development of recombinant drugs like insulin and factor VIII. Genetic engineering has advanced to an extent that it is now being performed in universities and high schools.

Gene therapy techniques include the insertion of a gene into a non-specific location. This technique is most common. The process of homologous recombination is used to replace abnormal genes with normal ones. Gene therapy also includes selective reverse mutation in which the abnormal gene can be made to return to its normal functioning.

However, in order to execute a successful gene transfer for medical purposes requires the meeting and maintenance of certain rigors. These include factors such as ensuring that the gene gets transferred to cells where it can function to its maximum potential; this varies with the disease to be treated. The transduced cells, the cells with the new genes, should survive and function normally for a long time and then be able to pass on the new gene so that the cure is long-lasting. The new genes must be able to produce enough factor to ensure blood clotting. The normal DNA should not be disturbed by the new transgenes. Avoidable aspects include irritating the cancer genes, a harsh immune response to the therapy, viral infection, etc.

The process of gene therapy involves the use of a carrier molecule or "vector" that acts as a delivery vehicle for the "normal" gene that is to replace the genes in the target cells. Viruses are commonly used as vectors as they naturally deliver pathogenic genes for propagation. In gene therapy, carrier viruses are genetically altered and made to carry human DNA. Common viruses used as gene therapy vectors include retroviruses, adenoviruses, adeno-associated viruses, and herpes simplex viruses.

Non-viral delivery systems are also used. One method is to inject the healthy gene directly into the desired cells. A drawback of this method is that it can be used only with certain tissues and the amount of DNA required is relatively large. Another approach calls for the creation of a liposome which carries the therapeutic DNA in its aqueous core. Its advantage is that it can pass the healthy genes through the membranes of the target cells. Another alternative, although less effective, is the chemical linking of the DNA to a molecule that binds to the cell receptors and from there the DNA is transferred through the cell membrane to inside the target cell. A very exciting technique involves the introduction of a 47th chromosome into the cells. This autonomous chromosome can act as a carrier of a large amount of genetic data; however it is difficult to deliver the large chromosome into the nucleus of the cells.

Gene therapy relies on two main transfer techniques. In vivo and ex vivo. With the in vivo technique, the cell's ability to absorb DNA is made use of. In the ex vivo method, the healthy gene is added to the cell outside the body. In vivo techniques when perfected will mean that a simple injection will enable the therapy. Ex vivo therapy involves more laboratory work and is laborious; it could be of use in individualized cases. It involves the removing of cells, growing of the transgenetic cells, analyzing the cells and finally adding them to the body. It is a customized approach that does not fit the mass production model.

There are several factors that have slowed down the progress of gene therapy. These include continuous procedures because of the short lifespan of the therapeutic effects of a single therapy procedure. Long-term benefits are hard to come by because it is not easy to integrate the healthy DNA into the cells, also the long-life and stability of the target cells cannot be guaranteed. The therapy stimulates the immune system and its response makes the therapy less effective. The option of using antibiotics to suppress the immune system can leave the patient vulnerable to disease. Viruses serve as the commonest vectors but they can turn toxic once inside the body and the body's inflammatory response to them can affect the procedure. Gene therapy has still not advanced to a stage where it can be used to treat multifactorial disorders such as high blood pressure, diabetes, Alzheimer's, etc.

Advances in gene therapy in the last few years include the ability to correct anomalies in messenger RNA obtained from defective genes. This has application in the treatment of cystic fibrosis and thalassaemia. It is now possible to create very small liposomes, only 25 nanometers across that can act as carriers of therapeutic DNA. Liposomes coated with polyethylene glycol have been used to transfer genes into the brain. This is a no mean achievement since the blood-brain barrier is too small to allow the passage of viral vectors. Gene therapy has been used to successfully treat sickle cell anemia in mice. An exciting new possibility has been created with the successful execution of gene silencing techniques; this may lead to the development of new ways to treat Huntington's. Metastatic cancer has been successfully treated using gene therapy. The procedure used killer T cells to target cancer cells. There have been encouraging reports about successful prevention of gene rejection by the immune system. microRNA has been used to help conceal the identity of healthy DNA to prevent their detection and destruction by the immune system.